The present invention relates to an aqueous ink for ink-jet printing which provides prints having excellent waterfastness and exhibits improvements in print density, ink dispersion stability, ink ejection stability and anti-clogging properties of the ink, and scratch resistance of printings.
Ink-jet printing is a non impact printing system in which ink droplets ejected from very small nozzles are directly adhered to a recording medium to form an image. Widespread ink-jet printing systems include a piezoelectric system applying a piezoelectric element to a printing head and a thermal-jet system utilizing the heating of a electrical resistive element. Ink developed for use in these systems generally comprises a water-soluble dye and a polyhydric alcohol as a wetting agent for the purpose of preventing clogging the nozzles. However, it has poor wvaterfastness. In particular, when used in a thermal-jet system, the water-soluble dye undergoes oxidation by excess heat on the printing head heater and makes kogation The terminology "kogation" as used herein is intended to mean that ink sticks and scorches on the heater.
In order to improve the waterfastness of the ink for ink-jet printing, it has been proposed to use ink comprising (a) a pigment (see Japanese Patent Laid-Open Nos. 28776/92, 189876/92, 359071/92, and 359072/92), (b) a non-aqueous liquid medium (see Japanese Patent Laid-Open No. 261478/92), (c) a dye having excellent waterfastness (see U.S. Pat. No. 4,963,189) or (d) emulsion or dispersion of polymer particles colored with a dye (see Japanese Patent Laid-Open Nos. 250069/91 and 340835/94).
However, use of a pigment in ink (proposal (a)) tends to incur a reduction in chroma of prints and to cause clogging of the nozzles. Further, the resulting prints have insufficient scratch resistance, namely, insufficient record preservability. Polymer particles (proposal (d)) are difficult to dye to a high degree. Even if dyed to a high degree, the polymer particles have poor stability, suffering from precipitation of the dye when left to stand for a long time. Neither have the proposals (b) and (c) succeeded in providing ink fully satisfying all the requirements, such as waterfastness and scratch resistance of prints and print density.
Additionally, it has been proposed to improve the waterfastness by using ink containing a polymer emulsion, such as ink containing a latex (see Japanese Patent Laid-Open No. 18412/80) and ink containing a colored polymer latex (see Japanese Patent Laid-Open No. 30873/84).
The problem associated with the ink containing latices is that a film is formed on the printing head to cause clogging. In order to inhibit film formation, a polyhydric alcohol is usually incorporated as a wetting agent. However, ink containing a polyhydric alcohol is unsuitable for application to the inkjet printing system for its too high viscosity. Further, the latex polymer to be used particularly for application to the thermaljet system should have anti-kogation properties.